The ability of light beams to rotate nano-objects has important applications in optical micromachines and biotechnology. However, due to the diffraction limit, it is challenging to rotate nanoparticles at subwavelength scale. Here, we propose a method to obtain controlled fast orbital rotation (i.e., circumgyration) at deep subwavelength scale, based on the nonlinear optical effect rather than sub-diffraction focusing. We experimentally demonstrate rotation of metallic nanoparticles with orbital radius of 71 nm, to our knowledge, the smallest orbital radius obtained by optical trapping thus far. The circumgyration frequency of particles in water can be more than 1 kHz. In addition, we use a femtosecond pulsed Gaussian beam rather than vortex beams in the experiment. Our study provides paradigms for nanoparticle manipulation beyond the diffraction limit, which will not only push toward possible applications in optically driven nanomachines, but also spur more fascinating research in nano-rheology, micro-fluid mechanics and biological applications at the nanoscale.

Particles trapped by optical tweezers, behaving as mechanical oscillators in an optomechanical system, have found tremendous applications in various disciplines and are still arousing research interest in frontier and fundamental physics. These optically trapped oscillators provide compact particle confinement and strong oscillator stiffness. But these features are limited by the size of the focused light spot of a laser beam, which is typically restricted by the optical diffraction limit. Here, we propose to build an optical potential well with fine features assisted by the nonlinearity of the particle material, which is independent of the optical diffraction limit. We show that the potential well shape can have super-oscillation-like features and a Fano-resonance-like phenomenon, and the width of the optical trap is far below the diffraction limit. A particle with nonlinearity trapped by an ordinary optical beam provides a new platform with a sub-diffraction potential well and can have applications in high-accuracy optical manipulation and high-precision metrology.